This invention relates to the non-destructive examination (such as ultrasonic examination) of stub tubes in the wall of a pressure vessel and particularly the stub tubes in the lower head of a nuclear reactor pressure vessel for receiving control rod drive housing tubes.
In a known type of nuclear reactor, for example as used in the Dresden Nuclear Power Station near Chicago, Illinois, the reactor core is of the heterogeneous type. That is, the core comprises a plurality of fuel assemblies vertically arranged in spaced array to form the nuclear reactor core capable of self-sustained nuclear fission reaction. The core is contained in a pressure vessel wherein it is submersed in a working fluid, such as light water, which serves both as a coolant and as a neutron moderator. A plurality of control rods, containing neutron absorbing material, are selectively insertable into the gaps or spaces between fuel assemblies to control the reactivity of the core.
Each fuel assembly comprises a tubular flow channel containing a fuel rod bundle formed of an array of elongated, cladded fuel elements or rods supported between upper and lower tie plates. The fuel assemblies are supported on control rod guide tube sockets in the pressure vessel between an upper core grid and a lower core plate. Each fuel assembly includes a nose piece which fits into the support socket and receives coolant from a pressurized coolant supply chamber. The pressurized coolant flows upward through the fuel assembly flow channel to remove heat from the fuel elements. A typical fuel assembly of this type is shown, for example, by D. A. Venier et al in U.S. Pat. No. 3,350,275.
A drive device for moving the control rods into and out of the core of the reactor is shown, for example, for R. R. Hobson et al in U.S. Pat. No. 3,020,887. As shown therein drive mechanism 36 is housed in a tube or thimble 38 which penetrates the lower head of the pressure vessel 10 and supports control rod guide tube 54.
As illustrated in this patent (FIG. 4), the control rod drive housing tube or thimble 38 extends through a hole in the pressure vessel 10 and is directly welded therein using welds 80 and 82.
However, according to common practice the housing tube is not welded directly to the pressure vessel. Instead, a stub tube is interposed between the housing tube and the pressure vessel, the stub tube being welded to the pressure vessel at its lower end and being welded at its upper end to the control rod drive housing tube extending therethrough.
Such stub penetrations thus become a part of the pressure vessel boundary and any defects (e.g., cracks) therein jeopardize the integrity of the pressure system.
It is desirable to provide non-destructive examination of the pressure system of nuclear reactors or the like to verify the integrity of the system or to discover any incipient defects so that appropriate repairs can be made before failure occurs.
A system for examining the walls of the pressure vessel and the seam welds therein is shown by J. P. Clark et al in U.S. Pat. Nos. 3,988,922 and 4,010,636. A system for examining vessel-to-nozzle and nozzle-to-pipe welds is shown by J. P. Clark et al in U.S. Pat. No. 3,934,457.
An object of this invention is to provide a system for the remotely controlled examination of the stub tube penetrations of a pressure vessel.
Another object is to scan successive radially spaced vertical paths on stub tubes in a pressure vessel.
Another object is an inspection device which readily can be placed in engagement with a stub tube penetration in a pressure vessel and which will automatically scan the outer surface of the stub tube upon initiation of a scanning operation.